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This paper shows the advances in the development of a portable system for remote detection of ammonium nitrate by nuclear quadrupole resonance. Three different probe coils were constructed and compared in terms of the signal intensity and the noise rejection under different environments, using the developed system. For the gradiometer probe coil, which showed the best performance in terms of noise rejection, the '90°' equivalent pulse was determined for different distances between the sample and the probe, and the remote detection of ammonium nitrate was achieved not only in a shielded room inside the laboratory, but also in the outdoors. The system was capable of detecting 200 g ammonium nitrate at 3 cm apart from the probe within 12 s with a steady-state free precession pulse sequence. The comparable results were obtained with a free induction decay sequence in 25 min for the total acquisition time. The compact size of the system will allow the applications for the substance detection in the outdoors especially for landmines made from ammonium nitrate and fuel oil mixtures instead of military explosives.

Nuclear quadrupole resonance (NQR) is a spectroscopic technique that can detect explosives of high chemical specificity and therefore it is very suitable for the landmine detection problem. There are several factors affecting the relation of noise signal ratio with NQR: molecular dynamics of relaxation, interference signals, thermal noise, explosive amount, distance, temperature and the design of the detecting system. Literature has searched ways for solving one or more of these factors, but since not all the work has been focused on landmine detection, many have only been tested in simulation or with data obtained under controlled laboratory conditions, where the detection is not remote (there is no separation between the sample and the detection system). This paper makes a review of the state of art in NQR, analyzing its relevance to the landmine detection problem and concluding about unsolved problems that could be the focus of future research.Keywords: Nuclear quadrupole resonance, landmine detection, explosive detection, landmine, demining. RESUMEN